1. Field of the Invention
The present invention relates to a single-phase motor driving circuit, and a single-phase motor driving method.
2. Description of the Related Art
For example, in a motor driving system disclosed in Japanese Utility Model Application Laid-Open Publication No. Hei3-74199, a comparator outputs a pulse signal (control signal) of “H” for driving a motor based on a saw-tooth wave voltage generated in accordance with a phase switching signal. A detected temperature voltage based on a temperature detected by a thermistor is inputted to the comparator as a comparison input relative to the saw-tooth wave voltage. That is, the comparator outputs the pulse signal of “H” in a period in which the saw-tooth wave voltage is larger than the detected temperature voltage.
The detected temperature voltage fluctuates in a certain range in accordance with a change in temperature detected by the thermistor. Thus, a width of the pulse signal of “H” outputted by the comparator is widened or narrowed in accordance with the change in the temperature detected by the thermistor. That is, the widening/narrowing of the width of the pulse signal can vary a revolution speed of the motor.
If a minimum value of the detected temperature voltage is larger than that of the saw-tooth wave voltage, a period in which an output of the comparator always becomes “L” is inevitably generated irrespective of the change in temperature detected by the thermistor, as shown in FIG. 4 of Japanese Utility Model Application Laid-Open Publication No. Hei3-74199. Consequently, the motor cannot be revolved at a full speed.
A particular situation is now assumed in which a temperature detected by the thermistor is low to increase a detected temperature voltage, and a width of a pulse signal of “H” outputted by the comparator becomes the narrowest. In this case, a maximum value of a detected temperature voltage is set large so that a revolution speed of the single-phase motor can be reduced more, i.e., a minimum revolution speed can be set as low as possible. Alternatively, a voltage for revolution at a lowest speed (lowest speed setting voltage) is set large in place of the detected temperature voltage. Then, when starting the single-phase motor in a stopped state (including restarting), the width of the pulse signal of the comparator is too narrow to obtain a necessary driving duty, and thus the motor cannot be started. Therefore, it is impossible to set low a minimum value of a revolution speed of the single-phase motor.